1. Field of the Invention
The present invention relates to an amplitude-phase correction circuit suitable for modulation/demodulation processing in the case where communication using multi-carrier signals is conducted, and also relates to a receiving device and a transmitting device to which such a correction circuit is applied.
2. Description of the Related Art
There is a communication system using a signal called a multi-carrier signal. In this system, a plurality of carriers are simultaneously transmitted, and information is distributed to each of the carriers to conduct communication, so that an efficient transmission becomes possible. As one of transmission systems using such multi-carrier signal, the present applicant proposed a transmission system using a signal having a configuration referred to as an orthogonal frequency division multiplex (OFDM) wave (as in Japanese laid-open patent publication No. 8-132434, and so on). In this transmission system, a time slot is formed by taking a predetermined time as a unit. In addition, band slots divided on the frequency axis are prescribed in the time slot. Processing of simultaneously transmitting a plurality of carrier signals located at a bandslot interval by using a predetermined number of band slots is conducted at a predetermined time slot period.
FIG. 1 is a diagram showing an example of configuration of a conventional receiving circuit of this OFDM wave. A signal 106 fed from an antenna or an amplifier (neither of them is illustrated) undergoes frequency conversion in a down converter 102 to produce an intermediate frequency signal or a baseband signal. A frequency converted signal 107 is supplied to a low-pass filter 103, in which a carrier component and so on are removed to produce an OFDM wave 108 which is a multi-carrier signal. This OFDM wave 108 is a multi-carrier signal containing a signal in a certain carrier interval (i.e., the above described band slot interval).
The OFDM wave 108 is converted to a digital signal in an analog/digital converter 104. Converted data are supplied to a fast Fourier transform circuit (hereafter referred to as FFT) 105. In the analog/digital converter 104, oversampling twice the number of carriers is conducted. The FFT 105 conducts processing for converting an inputted signal 109 formed in a time sequence to a signal 110 on a frequency sequence. The signal 110 on the frequency sequence is selected by a signal selection circuit 101. Since oversampling of a doubled rate is now conducted in the analog/digital converter 104, processing of selecting information contained in half carriers of the output from of the FFT 105 is conducted. A selected signal 111 is used as a received symbol sequence.
The state of the signal received by the circuit of FIG. 1 is shown in FIGS. 2A to 2E. In FIGS. 2A to 2E, FIG. 2A shows waveforms of values of a real part and an imaginary part, the amplitude, and the phase of a signal which should be inputted to the FFT 105 in an ideal state. The QFDM wave is supposed to be subjected to a QPSK modulation. In this case, the amplitude of each of the carriers is equal to each other and four steps of phase (3/4.pi., 1/4.pi., -1/4.pi., -3/4.pi.) are present irrespective of transmitted information. Phase values other than the four steps are not present.
In fact, however, dispersion occurs in the amplitude characteristics of the low-pass filter 103 in its pass band as shown in FIG. 2B. In addition, the phase rotates as shown in FIG. 2B. In general, the sharper the rising edges of the pass band and the cut-off band of a filter are set, the more significantly such changes of the amplitude and the phase occur in the pass band characteristic of a filter.
As a result, an actual signal inputted to the FFT circuit 105 after passing through the low-pass filter 103 becomes as shown in FIG. 2C, in which the amplitude is not constant and also as to the phase, phase values other than the four steps are present. If it is assumed that this signal is subjected to oversampling at a doubled rate in the analog/digital converter 104, it becomes as shown in FIG. 2D. As shown in FIG. 2E, the signal selected by the signal selection circuit 101 differs significantly from the ideal signal shown in FIG. 2A.
If there is such a signal change, transmitted information cannot be received accurately. Especially in the case where such communication as to transmit information by using phase differences among respective carries as a multi-carrier signal is conducted, the bad influence appears significantly.
In the receiving circuit, the amplitude and phase of a signal wave in the foregoing description are represented by handling the signal wave in the form of a real part component and an imaginary part component. Denoting the amplitude by r, the phase by .theta., the real part component by re, and the imaginary part component by im, representation conversion conducted at that time becomes re=r cos .theta., im=r sin .theta..